Air Movement & Flow

What it is: How the air moves and flows through a building provides important information for the movement of air pollutants.

Sources in Schools: Air movement in schools is generally caused by heating, ventilation and air conditioning systems (HVAC) ducts. The building itself affects how air is circulated within it, including the walls, doors, windows, and occupants of the building.

Devices & Measures

Air movement and flow can be measured with devices such as manometers, or anemometers.

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Manometer

Manometers are devices that are used to measure pressure. Water or some other fluid is placed in a U shaped tube separating a sealed container of gas and the gas in the atmosphere. A difference in pressure between the two gasses can be measured by observing the change in height of the liquid.

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Pressure

Pressure (P) is a measure of force (F) applied to a surface area (A). Force is measured in Newtons (N). Area is usually measured in square meters (m2) so pressure can be measured in N/m2 which is known as a Pascal (Pa).  Other common pressure units of measure include pounds per square inch (psi), millibars, atmospheres (atm), millimeters of mercury (mm Hg), and inches of water (in H2O).

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Measuring Pressure with a Manometer

To use a manometer to measure the pressure (P) the difference in the pressure in the manometer (P1) and the pressure outside the manometer (P2) can be computed by using the density of the liquid in the manometer (ρ), the gravitational constant (g) and the height (h = h2-h1). 

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Pitot Tube Manometer

A Pitot tube is a type of manometer that can be inserted into an air duct with the tip pointing towards the air flow and will measure the velocity pressure of the air.

Convert Velocity Pressure to Velocity

To convert the velocity pressure into the actual velocity of the air, the above equation is needed. Multiply 4005 by the square root of the change in pressure (ΔP measured in inches of water column - in WC; essentially the height of liquid change) to get the velocity (V in feet per minute - ft/min).

Air Flow

Multiplying air velocity (V) by the area of a duct (A) determines the air flow rate (Q) past a point in the duct during specified unit of time. Volume flow is typically measured in cubic feet per minute (CFM).